I was at a reading for Scott Westerfeld’s Leviathan when he mentioned off-hand that it would be a trilogy… with an illustrated guide to the world he was building, in the style of the Arthur Spiderwick’s Field Guide to the Fantastical World.
Now, there are a lot of reasons that I liked the Spiderwick guide—I’m a big fan of Tony DiTerlizzi, for instance—but the deep reason is that I’m gonzo for apocrypha. Those sorts of bits and extras that deepen worldbuilding, whether they are art books like Avatar: The Last Airbender: The Art of the Animated Series or in-world mythology like The Tales of Beedle the Bard. The icing on the cake with The Manual of Aeronautics is that Keith Thompson does the art for it, as he did for the series.
[A review]